Restore Maniphest subscriber transaction mail tag

Summary: Fixes T6748. This just didn't get aligned when CCs became a modern transaction.

Test Plan: Added a CC to a task, used bin/mail show-outbound to verify it showed up as a CC tag.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T6748

Differential Revision: https://secure.phabricator.com/D10991